Das Verhalten gasförmiger Alkalien in Kohlenstaubfeuerungen - experimentelle und thermodynamische Betrachtungsweise

Dissertation von Heiko Schürmann
Universität Stuttgart, 2011

The worldwide growing demand for heat and electrical energy in the near future will be amongst others warranted by the combustion of solid, fossil fuels. On this account the increase of power plant efficiency and the decrease of operational problems are required to save costs, fuels and in this way to protect the environment.
Alkali metals in fuels as a natural part of the mineral fuel content are responsible for many operational problems in combustion systems, like high temperature corrosion, fouling and slagging.

A broad set of data for the understanding of the behaviour of gaseous alkali metals in coal dust firing systems does not exist and for this reason a global theoretical and experimental examination was carried out in the present work. Absolute gaseous alkali concentrations were determined in-situ and in real time in the flue gas of a semi-technical pulverised coal combustor by excimer laser fragmentation fluorescence (ELIF). The combustor was operated at atmospheric pressure in the temperature range of 1000-1300°C. While firing different coals, the effects of varying combustor temperature, residence time, excess air (λ), coal mineral content and flue gas composition (content of chlorine, sulphur and water) were investigated. ELIF data of absolute gaseous alkali concentration were then used together with modified fuel analyses and thermodynamic equilibrium calculation to obtain a more detailed understanding of alkali release and capture behaviour under pulverised coal combustion conditions.

Results obtained for release of Potassium (K) and Sodium (Na) show that qualitatively, the two species behave similarly. Quantitatively, however, there are significant differences, largely resulting from their different affinities towards minerals in the fuel. A strong dependence of increasing gaseous alkali metal concentration in flue gas with increasing temperature, especially when the softening point of coal ash was reached, could be observed. Increasing the residence time of the flue gas in the reactor leads to a drop of gaseous alkali metals. The enlarged reaction time between the gaseous alkali species with fly ash particles facilitating the formation of solid alkalialuminosilicates are responsible for this effect. Further on, it could be observed that the main reaction of alkali release from coal took place immediately (< 1 second) after entrance of the coal to the reactor.
Higher air numbers of combustion leads to higher alkali metal concentrations in the flue gas because of an intense reaction of the coal particles with oxygen and hence higher temperatures at the coal particle.
The addition of small portions of clay to the combustion process leads to a decrease of gaseous alkali species because of the formation of solid alkali mineral phases. Two reactions are responsible for this effect. From the chemical point of view a larger amount of acid ash components, like silicon and aluminium support the reaction between them and the basic alkali metals to form solid alkalialuminosilicates. From the physical point of view the concentration of fly ash particles in flue gas was higher and the probability of contact therefore increases considerably.

Chlorine was identified as main influence parameter on alkali release. The addition of defined amounts of vaporised HCl to the flue gas led to a dramatic increase of gaseous sodium and potassium concentrations. The addition of vaporised H2SO4 was converse to the addition of HCl. The higher amount of sulphur led to a decrease of gaseous alkali metals in flue gas. The formation of alkali sulphates, which couldn’t be measured by ELIF, was found as explanation.
Then, with increasing residence time a higher concentration of gaseous alkali species was measured. The resulting curve reaches a maximum, whereas it was assumed, that the alkali sulphates converted to measurable alkali chlorides. Afterwards a decrease of the gaseous sodium and potassium concentration in flue gas was measured, because of capture reactions with fly ash particles. As a conclusion of this behaviour it was assumed that the formation of solid alkalialuminosilicates is inhibited by alkali sulphates and favoured by alkali chlorides.
Finally, the effect of water vapour was tested. It was found that the presence of increasing shares of water vapour in flue gas supports the formation of solid alkalialuminosilicates and decreased in this way the concentration of gaseous alkali metals in the flue gas.

For further description of the behaviour of alkali metals in high temperature combustion processes thermodynamic equilibrium calculations were carried out. A good accordance between calculated and measured values was found. The higher affinity of potassium to form solid mineral phases and of sodium to be released to the gaseous phase could be confirmed. As well the measured trends in dependence to temperature, air number, coal ash composition, influence of chlorine and water vapour could be confirmed too.

Furthermore, the chemical fractionation of coal was used for evaluation of the behaviour of alkali metals in flue gas. Coal samples were extracted by water, ammonia acetate solution and HCl solution. The analysis of the alkali metal content in the different extraction solutions and the comparison to the remaining alkali metals in coal led to the prediction that the solvable alkali metals are volatilised during combustion. The comparison of measured values of gaseous alkali metals in combustion flue gas with values of soluble alkali metals was found in good accordance. As a result, it could be recommended to carry out the additional analysis step of chemical fractionation to get an overview about volatilised alkali metals during combustion.

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